This invention relates to the field of DC/DC converters and associated frequency trim circuitry therefor and, more particularly to a converter which operates efficiently in a communication receiver environment at frequencies below the intermediate frequency (IF) of the communication receiver without initiating "self-quieting" of the radio.
Presently, DC/DC converters are widely used in systems requiring voltages higher than the power supply or battery can provide. In particular, DC/DC converters are now commonly used in radio paging receivers to generate a 3 volts supply from a one cell 1.5 volt battery in order to provide for the proper operation of a CMOS microcomputer decoder circuit.
In the past, the use of coil-type DC/DC converters in communication receivers has caused interference and desensitization problems that could only be solved by restricting the operating frequency of the converter to either very low audio frequencies (&lt;10 KHz), or to frequencies above the final IF or intermediate frequency used in the receiver. The interference and desensitization are caused by harmonic signals that are generated by the switching waveforms within the DC/DC converter, and are coupled into the amplifier stages of the receiver by various stray capacitive and inductive coupling mechanisms. Limiting the operating frequency of the converter to frequencies above the final IF insures that the interference signals generated by the converter will all be at frequencies that lie above the final IF, and thus there will be no interference. Similarly, limiting the operation of the converter to very low frequencies insures that only very high level harmonics are potential sources of interference and, since the energy in the harmonics of the switching signal decreases rapidly with increasing harmonic frequencies the level of interference is effectively minimized.
The prior art restrictions on the operating frequencies of DC/DC converters directly impact the size and performance of the converter. In particular, the optimum converter efficiency that can be achieved with the physically small coils that must be used in paging receivers is obtained for operating frequencies in the range from 50 to 150 KHz. Since this desired range of operating frequencies is harmonically "close" to the 455 KHz IF frequency that is commonly used in receivers, it has become desirable to provide a technique for using DC/DC converters in communication receivers that allows the converter to operate at frequencies that are low order harmonic sub-multiples of the IF frequency.